Direct Optimization of Fast-Ion Confinement in Stellarators

TL;DR

This paper presents a method to directly optimize stellarator designs for better alpha particle confinement by simulating trajectories, bypassing proxy metrics, despite high computational costs.

Contribution

It introduces a direct trajectory-based optimization approach for stellarator design, improving alpha confinement without relying on simplified proxy metrics.

Findings

Successfully generated configurations with low alpha particle losses.

Demonstrated the feasibility of direct trajectory optimization despite computational expense.

Provided insights into the trade-offs of proxy metrics versus direct simulation.

Abstract

Confining energetic ions such as alpha particles is a prime concern in the design of stellarators. However, directly measuring alpha confinement through numerical simulation of guiding-center trajectories has been considered to be too computationally expensive and noisy to include in the design loop, and instead has been most often used only as a tool to assess stellarator designs post hoc. In its place, proxy metrics, simplified measures of confinement, have often been used to design configurations because they are computationally more tractable and have been shown to be effective. Despite the success of proxies, it is unclear what is being sacrificed by using them to design the device rather than relying on direct trajectory calculations. In this study, we optimize stellarator designs for improved alpha particle confinement without the use of proxy metrics. In particular, we numerically optimize an objective function that measures alpha particle losses by simulating alpha particle trajectories. While this method is computationally expensive, we find that it can be used successfully to generate configurations with low losses.